As is well-known, the production of aromatic hydrocarbons is greatly interested for the whole chemical industry. For that purpose, rather simple processes exist, in particular, for the purpose of separating aromatic from non-aromatic species; the resulting mixture of aromatic species is easily separated by distillation into C.sub.6 aromatics (benzene) with a suitable purity for any possible use, and into C.sub.7 aromatics (toluene), also with a high purity level.
The situation is completely different as regards C.sub.8 aromatics, the mixtures of which not always easily be separated, because they are constituted by mixtures of isomers with very close boiling points. The most interesting C.sub.8 mixture from the industrial viewpoint is that mixture which is constituted by ethylbenzene, o-xylene, p-xylene and m-xylene: as such, said mixture is used as a solvent or in gasolines, whilst much wider would be the range of possible uses of each individual isomer, if each could be separated with high purity level. So, a demand exists for high-purity para-xylene for preparing terephthalic acid and dimethyl terephthalate, intermediates for man-made fibre synthesis. Ethylbenzene, in turn, is an interesting intermediate in styrene preparation by dehydrogenation.
Usually, the processes used heretofore in order to separate the above said isomers have been mostly based on suitable combinations of superfractionation techniques and of low-temperature crystallation processes: all the above having high costs and limited yields.
According to alternative routes, the para-isomer is separated by adsorption, or by using molecular sieves: the various isomers can then be isomerized and the produced para-xylene can be separated again according to the above mentioned technique.
The ortho- and ethylbenzene isomers can also be separated by distillation.
However, the above mentioned para-isomer separation process by crystallization does not make it possible for said isomers to be totally recovered; the presence of the product in the mother liquors sent to the isomerization reduces the isomerization yield per pass, and increases the need for recycles.
Furthermore, the presence of ethylbenzene causes even more serious problems, because this produce tends to accumulate in the isomerization step, causing still more serious recycling problems. If possible, then, its conversion into dimethyl benzene always takes place with low yields and serious operation problems.
From U.S. Pat. No. 3,698,157, a process is known as well for recovering para-xylene from a mixture of C.sub.8 aromatic by contacting said mixture with a suitably modified zeolite. Also the formation is envisaged of two fractions constituted by:
______________________________________ meta Pboil: 139.12.degree. C. .vertline. para Pboil: 138.37.degree. C. ortho Pboil: 144.41.degree. C. .vertline. ethyl-b Pboil: 136.19.degree. C. ______________________________________
Such fractions can then be separated by distillation. The meta/ortho mixture can yield the meta-isomer as the overhead fraction and the ortho-isomer as the bottom fraction, with reflux ratios of the order of 10-20, and with a number of trays comprises within the range of from 150 to 200.
The para/ethyl-b mixture can also be separated by distillation, under burdensome conditions (however, obviously, not as burdensome as they would be if the whole stream were submitted to distillation).